The present invention relates to optical switches and, more particularly, to crossbar switching components for optical signal processing, and to methods of improving optical signal processing by means of such crossbar switching components.
Although fiberoptic systems have been the backbone of the telecommunications industry, their main use to date has been as point-to-point information pipelines. The switching and processing of signals have been accomplished using electronic devices.
Optical cross-connect (OXC) switches are important in optical fiber communications networks, particularly in systems using wavelength division multiplexing (WDM). Optical switches can be divided into two classes, all-optical and optoelectronic. All-optical switches are distinguished by the lack of any electrical conversion of the optical signals being switched. This enables them to be bit rate independent and protocol transparent. In addition, the lack of electrical conversion allows for support of bit rates ranging up to 40 Gb/s and higher, beyond the reach of practical optoelectronic systems.
Crossbar interconnection networks are essential components in a variety of optical signal processing applications, such as communication signal switching and parallel computation. Such applications are described in articles such as that by M. Fukui entitled “Optoelectronic Parallel Computing System With Optical Image Crossbar Switch”, in Applied Optics, Vol. 32, pp.6475-6481 (1993), and that by Y. Wu, L. Liu, and Z. Wang, entitled “Optical Crossbar Elements Used For Switching Networks”, published in Applied Optics, Vol. 33, pp. 175-178 (1994).
Such crossbar interconnection networks have been incorporated into optical configurations for performing dynamic vector matrix multiplication and arbitrary interconnection between N inputs and M outputs. They take advantage of the speed and the parallelism of optical signal transmission to provide performance levels significantly better than these attainable using microelectronic devices. Several such systems, using discrete components and free space propagation, have been described in the literature, such as by J. W. Goodman, A. R. Dias, and L. M. Woody in “Fully Parallel, High-Speed Incoherent Optical Method For Perfoming Discrete Fourier Transforms”, published in Optics Letters, Vol. 2, pp. 1-3 (1978); and by M. Fukui and K. Kitayama in “High-Throughput Image Crossbar Switch That Uses A Point Light Source Array”, published in Optics Letters, Vol. 18, pp.376-378 (1993).
Such configurations typically consist of a number of conventional lenses and a dynamic spatial light modulator (SLM). The use of discrete optical components, and the relatively large number thereof required, results in a switch with comparatively high weight and volume. Furthermore, the individual components have to be mounted and aligned mechanically. The alignment accuracy required between individual components is extremely difficult to achieve and often impractical for the high density of channels to be switched. As a result, free space propagation optical crossbar switching systems are very sensitive and non-robust, have relatively low positioning accuracy and are subject to thermal instability, thus making them unsuitable for general industrial use.
All of these factors combine to make such free space switches incompatible with the small size and circuit construction techniques used in the integrated optoelectronic technology used in modern signal processing and communications systems. There is, therefore, need for an optical crossbar switch that combines the speed of optical processing techniques with the small dimensions typical of microelectronic technology.
The problems associated with discrete elements and free space configurations can be alleviated by using planar optics configurations, in which several optical elements (lenses, filters, beam splitters, polarizers, etc.) can be integrated onto a single substrate. The light propagates between the different optical elements, inside the substrate, either by total internal reflection or with the aid of reflective coatings on the substrate surfaces. The alignment of several optical elements integrated onto one substrate can be done with relatively high accuracy during the recording of the elements in the laboratory. Planar optical technology is fully compatible with microelectronic detectors, devices and production technology, with the element patterns being generated by standard microelectronic production techniques such as photolithography and etching.
After surveying the prior art, PCT International Publication No. WO 98/33335 to Reinborn et al. submits that there is no known method wherein an optical crossbar switch can be implemented using planar technology.
PCT International Publication No. WO 98/33335 subsequently discloses an optical crossbar switch with performance typical of conventional bulk component switches, but with a level of compactness compatible with optoelectronic circuit package sizes. The planar optical crossbar switch has two thin planar substrates made of an optical medium such as glass, on each of which is recorded or attached two holographic lenses. The light propagates inside each substrate between the two lenses by means of total internal reflection, or if the surfaces of the substrates are coated with a reflecting layer, by means of specular reflection. The two lenses on each substrate are disposed either on opposite sides of the substrate, or on the same side of the substrate, depending on the optical configuration used. The first lens is a negative cylindrical lens, used to input the incident light signal to the substrate. If this light source is in the form of a linear array, it can be positioned on top of the lens. The second lens is a positive cylindrical lens. The two substrates are disposed at right angles to each other with their positive lenses disposed one on top of the other. A two dimensional array of binary optical switches, acting as a spatial light modulator (SLM), is sandwiched between these two positive lenses, and spatially modulates the light transmission passing through it.
FIG. 1A of the prior art, as disclosed by PCT International Publication No. WO 98/33335, illustrates the input substrate of a planar optical crossbar switch. The substrate 10 is constructed of a transparent optical medium such as glass. A negative holographic cylindrical lens HL1(−) 12 is recorded onto the substrate, or attached thereto, and a linear array 14 of light emitting sources 16, such as light emitting diodes (LED's), is disposed above this lens, such that the light from each source in the array is coupled into the substrate by the lens. Once inside the substrate, the light is trapped by total internal reflection 18, or if the surfaces of the substrate are coated with a reflecting layer, by means of specular reflection. A second holographic lens HL1(+) 20, in this case a positive cylindrical lens, is recorded on the substrate, or affixed to it at a point distant from the first lens, such that the light fanning out from HL1(+) is collimated by HL1(+) and coupled out of the substrate 22. HL1(+) can be disposed on the same side of the substrate as HL1(−), or on the opposite side.
FIG. 1B of the above-referenced patent application publication shows an output substrate 30 of the planar optical crossbar switch. The constriction of this substrate is identical to that of the input substrate, in that it has a positive holographic cylindrical lens HL2(+) 32 at one position of its surface, and a negative holographic cylindrical lens HL2(−) 34 at another. In this second substrate, however, the direction of propagation of the light is opposite, i.e., from HL2(+) to HL2(−). Furthermore, at the output of HL2(−), a linear detector array 36 is disposed, operative to detect the light collected by HL2(−). In FIG. 1A, this detector is drawn at a distance from HL2(−), but it can preferably also be disposed close to or in contact with the substrate.
FIG. 2 shows a complete planar optical crossbar switch, formed by combining two substrates as shown in FIGS. 1A and 1B. The substrates 10, and 30 are aligned at right angles to each other, with their respective positive holographic lenses 20 and 32, aligned one on top of the other, but rotated 90 degrees to each other. A thin planar pixelated spatial light modulator 38, such as a ferroelectric liquid crystal device, is disposed between them. The interposed SLM and substrates can be attached together into one rugged unit, so as to effectively form one continuous substrate assembly. The crossbar switch function is realized by configuring the lenses such that on the input substrate 10, the light from a particular element of the linear source array 14 is spread out across a particular row of the SLM matrix, while on the output substrate 30, since it is rotated to be at right angles to the input substrate, the lenses are operative to converge the light from a particular column of the SLM matrix onto a particular element of the linear output detector array 36.
The above-described device has several distinct disadvantages. A single light source is diffracted by a negative lens, such that the light emitted from the device is spread out along a line (corresponding to a switching matrix array). PCT International Publication No. WO 98/33335 further discloses that the interconnection matrix between arbitrary elements at the input array of the switch to elements of the output detector array is determined by which particular elements of the SLM matrix are on or off. For example, to connect a signal from the ith source in the input array to the jth detector in the output array, the value of the {i,j} pixel of the SLM matrix should be on, i.e., this pixel should be in the transparent state if a transmissive SLM is being used, or in the predetermined reflective state if a reflective SLM is being used.
Thus, the light emanating from the light source is dispersed, while only the light from a singe point (pixel), which corresponds to a small fraction of the input from the light source, actually passes through the switch and for collection and routing on the other side. The remainder of the dispersed light is blocked by the ‘off’ elements in the SLM matrix. Consequently, despite the advantages inherent in the above-described all-optical switch, the energy inefficiency makes such a switch of very limited practical utility. The problem is particularly severe for large switching arrays, in which the ratio of light output to light input can easily fall below 0.1. Furthermore, the light intensity is not uniform across each row of the SLM matrix, mid therefore light output may vary according to the choice of the {i,j} connection.
There is therefore a recognized need for, and it would be highly advantageous to have, an all-optical switch that is simple and reliable, like the switch disclosed in PCT International Publication No. WO 98/33335, but is significantly more efficient and controlled from an energy standpoint.